Energy used by humans comes from many different sources. Fuels like wood have long been used to heat dwellings and cook food. Recently, fossil fuels have been widely used to power vehicles, heat homes, and to produce electrical energy for myriad modern technologies and conveniences. Such fuels create numerous challenges, however, as the earth may hold only a limited supply, and burning them for our energy needs may produce pollution and may contribute to climate change. However, renewable energy sources, such as solar energy, may supplement or replace fossil fuels (or other types of fuel) without the deleterious effects of those fuels.